Surface-Enhanced Raman Spectroscopic Investigation of PAHs at a Fe3O4@GO@Ag@PDA Composite Substrates

Abstract

A method for surface-enhanced Raman spectroscopy (SERS) sensing of polycyclic aromatic hydrocarbons (PAHs) is reported. Fe₃O₄@PDA@Ag@GO is developed as the SERS substrate prepared by classical electrostatic attraction method based on the enrichment of organic compounds by graphene oxide (GO) and polydopamine (PDA) and the good separation and enrichment function of Fe₃O₄. The morphology and structure of the SERS substrate were represented by transmission electron microscopy (TEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD) and the UV-visible absorption spectrum (UV-vis spectra). The effect of different temperatures on SERS during synthesis was investigated, and it was found that the best effect was achieved when the synthesis temperature was 90 °C. The effect of each component of Fe₃O₄@PDA@Ag@GO nanocomposites on SERS was explored, and it was found that Ag NPs are of great significance to enhance the Raman signal based on the electromagnetic enhancement mechanism; apart from enriching the polycyclic aromatic hydrocarbons (PAHs) through π-π interaction, GO also generates strong chemical enhancement to the Raman signal, and PDA can prevent Ag from shedding and agglomeration. The existence of Fe₃O₄ is favored for the fast separation of substrate from the solutions, which greatly simplifies the detection procedure and facilitates the cycle use of the substrate. The experimental procedure is simplified, and the substrate is reused easily. Three kinds of PAHs (phenanthrene, pyrene and benzanthene) are employed as probe molecules to verify the performance of the composite SERS substrate. The results show that the limit of detection (LOD) of phenanthrene pyrene and benzanthene detected by Fe₃O₄@PDA@Ag@GO composite substrate are 10^-8 g/L (5.6 × 10^-11 mol/L), 10^-7 g/L (4.9 × 10^-10 mol/L) and 10^-7 g/L (4.4 × 10^-10 mol/L), respectively, which is much lower than that of ordinary Raman, and it is promising for its application in the enrichment detection of trace PAHs in the environment.

Keywords: Ag; PAHs; composite substrate; enrichment detection; surface-enhanced Raman spectroscopy.

Figure 1

UV absorption spectra of silver dissolved at different temperatures.

Figure 2

TEM images Fe₃O₄@PDA@Ag@GO of different magnifications. (1) TEM image zoomed to 1 μm; (2) TEM image zoomed to 500 nm; (3) TEM image zoomed to 200 nm; (4) TEM image zoomed to 100 nm.

Figure 3

TEM mapping and EDS of Fe₃O₄@PDA@Ag@GO nanocomposites. (1) The elemental mappings of C; (2) the elemental mappings of Ag; (3) the elemental mappings of O; (4) the elemental mappings of Fe; (5) the elemental mappings of N; (6) EDS of Fe₃O₄@PDA/Ag/GO.

Figure 4

XRD of Fe₃O₄@PDA@Ag@GO.

Figure 5

SERS spectra of phenanthrene (1) phenanthrene solid; (2) the mixture of 10⁻² g/L phenanthrene and SERS substrate solution; (3) Fe₃O₄@PDA@Ag@GO solution; (4) 10⁻² g/L phenanthrene solution.

Figure 6

SERS signal of phenanthrene captured on Fe₃O₄@PDA@Ag@GO, Fe₃O₄@PDA@Ag, Fe₃O₄@PDA and Fe₃O₄, respectively (1) Fe₃O₄@PDA@Ag@GO substrate; (2) Fe₃O₄@PDA@Ag substrate; (3) Fe₃O₄@PDA substrate; (4) Fe₃O₄ substrate.

Figure 7

SERS spectra of different mixing volume ratios of Fe₃O₄@PDA@Ag@GO substrate and phenanthrene solution (1) 2:1;(2) 1:1; (3) 1:2; (4) 1:4.

Figure 8

The detection limit of phenanthrene captured on Fe₃O₄@PDA@Ag@GO nanocomposites SERS substrate (1) phenanthrene solid; (2) 10⁻⁵ g/L phenanthrene SERS solution; (3) 10⁻⁶ g/L phenanthrene SERS solution; (4) 10⁻⁷ g/L phenanthrene SERS solution; (5) 10⁻⁸ g/L phenanthrene SERS solution.

Figure 9

Linear relationships between the phenanthrene characteristic intensity and the logarithm of phenanthrene concentration. (a) 590 cm⁻¹; (b) 411 cm⁻¹.

Figure 10

The detection limit of Pyrene captured on Fe₃O₄@PDA@Ag@GO nanocomposites SERS substrate (1) Pyrene solid; (2) 10⁻⁵ g/L Pyrene SERS solution; (3) 10⁻⁶ g/L Pyrene SERS solution; (4) 10⁻⁷ g/L Pyrene SERS solution; (5) 10⁻⁸ g/L Pyrene SERS solution.

Figure 11

Linear relationships between the pyrene characteristic intensity and the logarithm of pyrene concentration. (a) 1016 cm⁻¹; (b) 590 cm⁻¹.

Figure 12

The detection limit of benzanthene captured on Fe₃O₄@PDA@Ag@GO nanocomposites SERS substrate (1) 10⁻⁴ g/L benzanthene standard solution; (2) benzanthene solid; (3) 10⁻⁵ g/L benzanthene standard solution; (4) 10⁻⁶ g/L benzanthene standard solution; (5) 10⁻⁷ g/L benzanthene standard solution; (6) 10⁻⁸ g/L benzanthene standard solution.

Figure 13

Linear relationships between the benzoanthracene characteristic intensity and the logarithm of benzoanthracene concentration. (a) 1033 cm⁻¹; (b) 356 cm⁻¹.